1. Field of the Invention
The present invention relates to circuits used for testing high frequency electrical systems, such as radio frequency RF transmitters and receivers; and more particularly to circuits used for detecting the presence of alternating current AC signals at test points in such circuits.
2. Description of Related Art
In a manufacturing environment, a means by which RF components can be quickly tested to determine whether the RF signal has reached the component is of significant value. A circuit which provides this test allows technicians with no RF experience and no special test equipment to perform testing quickly and cheaply. Ideally, such an RF sensing circuit would have a digital output that indicates the presence of RF energy at its input. This digital output could then be connected with high speed automated test equipment for hands off testing.
FIG. 1 illustrates one prior art AC signal detector which is useful at RF frequencies. The test point 10 is connected in the circuit of FIG. 1 through a DC blocking capacitor 11, which passes the AC signal from the test point line 10 to the point 12. A diode 13 is used to half wave rectify the incoming signal at node 12. The rectified signal is passed through an LC filter comprising inductor 14 and capacitor 15. The output of the filter is a DC voltage at node 16 which is supplied to the input of a finite gain amplifier 17. A resistor 18 is connected from node 16 to the supply potential to provide a load on the circuit. A reference circuit is provided with resistor 20 and diode 21 connected in series. The node between the diode 21 and the resistor 20 is connected to the negative input of the amplifier 17. The amplifier 17 produces an output proportional to the RF signal level at the input 10. In some versions of this circuit, the amplifier 17 can be replaced with a voltage comparator whose output goes high when the RF signal exceeds a pre-determined level above the reference voltage, and thereby provides a digital output. One problem with the circuit of FIG. 1 is that it requires a relatively large inductor 14. This makes the circuit impractical for use in an integrated circuit environment. Also, the low impedance input 10 unduly loads the circuit being tested, unless isolation circuits like directional couplers are used. Even with directional couplers, the input to the detector would be substantially reduced in magnitude, limiting the sensitivity of the detector.
FIG. 2 illustrates an alternative approach to RF detection. The circuit in FIG. 2 is referred to as bolometer detector. An RF input at node 30 is connected across a transmission line 31 having a characteristic impedance of for example 50 ohms. A detection resistor 32 is coupled to the transmission line 31. As the current in the resistor 32 increases, its temperature increases. A temperature-to-voltage converter 33 is coupled with the resistor 32 and generates an output that is connected to amplifier 34. A reference circuit is provided by resistor 35 which is connected to ground on both ends. The temperature of the resistor 35 is converted to a voltage by converter 36. The output of the converter 36 is connected to the amplifier 34. The amplifier 34 generates an output on line 37 that measures the root mean square RMS power of the RF signal on the input 30. As with the circuit of FIG. 1, the amplifier 34 could be replaced with a comparator that provides a digital output on line 37.
The bolometer detector is an excellent RF signal detector. However, it suffers the disadvantage that it loads the input 30 significantly. Thus, for very small signals, the bolometer detector has too much impact on the circuit being tested to provide useful results.
Accordingly, it is desirable to provide an RF detector circuit which presents a high impedance to the circuit under test over a broad range of frequencies, and minimally loads the circuit. Furthermore, it is desirable that the detector be AC coupled to the circuit under test so as not to upset or change the bias conditions of the circuit. Furthermore, such detectors should be physically small, and suitable for implementation on an integrated circuit so that the cost of the detector is minimized and the detector takes up very little board space on the circuit being tested. In addition, the detector should be low cost, and consume very little power in the circuit under test or in the circuit which carries the detector.